Parsing JSON into Python
Try it: Parsing JSON into Python
How json.loads() reads JSON text token by token and builds Python dicts, lists, strings, numbers, True/False and None, how an access path like data["users"][0]["name"] walks that structure, and what json.dumps() writes back.
How it works
- The scanner reads the next token: a brace or bracket, a string in double quotes, a number, true / false / null, or a ',' / ':' delimiter.
- '{' starts a Python dict and '[' a list; both are pushed on a parse stack until their closing '}' or ']'.
- Inside a dict every key must be a double-quoted string followed by ':' and a value; items are separated by ','.
- Anything else stops the parse with JSONDecodeError, reporting the message, line, column and character position.
- The access path is applied one subscript at a time: dicts take keys (KeyError if missing), lists take integer indexes (IndexError if out of range).
- json.dumps() turns the Python objects back into JSON text (True → true, None → null), optionally indented.
Default run (43 steps): json.loads() receives 105 characters of text. The scanner starts at char 0 and reads one token at a time. … json.dumps(data, indent=2) turns the Python objects back into JSON text (171 characters): True → true, None → null, and strings get double quotes.
Simplified: Written from scratch to match CPython 3.12's json module (messages and positions checked against it); input is limited to 400 characters and 24 nesting levels, and the access path accepts only ["key"] and [index] subscripts. Python 3.13 reports a trailing comma in an object with a different message.
Loading the simulation…